An acoustic source radiates energy into its surroundings. If this source is an engineered loudspeaker, its radiated energy has an envelope shaped to present uniform energy to the audience. The ability of a loudspeaker to control its radiated energy in this way is diminished at lower frequencies, where wavelengths are larger than the loudspeaker itself, and acoustic energy radiates in all directions equally. In this case, the loudspeaker is said to be omnidirectional.
A surface mounted loudspeaker generates two distinct acoustic energy arrivals, one direct from the transducer and the other reflected from the surface to which it is mounted. The interference of the reflected energy with the direct energy is primarily destructive by creating dramatic frequency response errors. The frequency of these errors is directly related to the time difference between the two energy arrivals at the listener.